1. Field of the Invention
The present invention relates to a magnetic disk to be mounted on a magnetic disk device such as a hard-disk drive (hereinafter abbreviated as HDD) and a lubricant compound used for a magnetic disk.
2. Description of Related Art
With the recent trend to higher-capacity information processing, various information recording technologies have been developed. Particularly, a surface recording density of an HDD using the magnetic recording technology has continuously increased by the rate of approximately 100% a year. In recent years, an information recording capacity exceeding 250 GB per disk is required for a magnetic disk having a diameter of 2.5 inches used in HDD or the like, and in order to meet such demand, realization of an information recording density exceeding 400 Gbits per 1 square inch is in demand. In order to achieve the high recording density in a magnetic disk used in an HDD or the like, magnetic crystal grains constituting a magnetic recording layer handling recording of an information signal need to be refined, and its layer thickness needs to be reduced at the same time. However, in the case of a magnetic disk of an in-plane magnetic recording method (also referred to as longitudinal magnetic recording method or horizontal magnetic recording method) which has been merchandized, as the result of development of the refining of the magnetic crystal grains, a thermal fluctuation phenomenon in which thermal stability of the recording signal is damaged by a superparamagnetic phenomenon and the recording signal is lost begins to occur, which makes an obstructive factor to higher recording density of a magnetic disk.
In order to solve this obstructive factor, a magnetic recording medium for a perpendicular magnetic recording method has been proposed recently. In the case of the perpendicular magnetic recording method, unlike the in-plane magnetic recording method, a magnetization easy axis of a magnetic recording layer is adjusted to be oriented in the perpendicular direction with respect to a substrate surface. As compared with the in-plane recording method, the perpendicular magnetic recording method can suppress the thermal fluctuation phenomenon, and this is suitable for higher recording density. This type of perpendicular magnetic recording mediums include a so-called two-layer type perpendicular magnetic recording disk provided with a soft magnetic underlayer made of a soft magnetic body on a substrate and a perpendicular magnetic recording layer made of a hard magnetic body as described in Japanese Unexamined Patent Application Publication No. 2002-74648, for example.
In a prior-art magnetic disk, a protective layer and a lubrication layer are provided on a magnetic recording layer formed on a substrate in order to ensure durability and reliability of the magnetic disk. Particularly, the lubrication layer used on the outermost surface requires various characteristics such as long-term stability, chemical substance resistance, friction property, heat resistant property and the like.
In order to accommodate such request, a perfluoropolyether lubricant having a hydroxyl group in the molecule has been widely used as a lubricant for a magnetic disk. For example, as in Japanese Patent Laid-Open No. 62-66417 (Patent Document 1), a magnetic recording medium in which a perfluoroalkylpolyether lubricant having a structure of HOCH2CF2O(C2F4O)p(CF2O)qCF2CH2OH having a hydroxyl group at both ends of the molecule is applied is well known. If there is a hydroxyl group in a molecule of a lubricant, it is known that adhesion properties of the lubricant to the protective layer can be obtained by means of an interaction between the protective layer and the hydroxyl group.